It is well known that a large number of concrete super-structures of the kind used for road or railroad bridges and which have been built a long time ago, are presently operating with considerably decreased coefficients of safety as compared to the safety coefficients of the original designs, because of causes that definitely diminish to a great extent the load capacity of concrete structures. Among such causes of failure, the following may be mentioned as the most important ones: the high increase in the movable loads that circulate over said bridges; the periodic but continuous stressing of the girders of bridges, which render the concrete materials more brittle, thus originating failures due to fatigue; the prolonged exposure of the tension steel rods and elements to the direct corrosive action of the environment; the vehicle accidents which cause impacts against the super-structures of bridges; and also the rather frequent constructional defects, including design failures.
It is also well known that many techniques for reinforcing or repairing concrete bridges have been developed in the past, among which the most important one, in view of the fact that it is very frequently used for reinforcing or repairing concrete bridges of different spans, is the technique that renders said bridges semi-continuous. Said well known technique comprises introducing compression elements between the headers of the girders, which permits the placement of a perpendicular supporting diaphragm at the center of the span of the girders such that it will project outwardly of the sides of the outer girders, in order to serve as a "violin strings bridge" for a plurality of tendons, the ends of which are attached to each girder, said tendons being passed over the upper edge of the supporting diaphragm, thus forming broken lines in vertical planes, and being finally anchored in foundation blocks that are placed behind the diaphragms of the buttresses. The steel cables or tendons are then introduced within polyethylene sheaths and are thereafter injected with cement slurry. This method, however, results very costly in view of the fact that all the reinforcement steel must be replaced, and is also very slow and requires relatively long lasting traffic interruptions which, added to the type of protection of the tendons, that renders their inspection practically impossible and their maintenance very difficult, together with the vulnerability of polyethylene against fire, rodents and environmental agents that age and destruct the same very quickly, and finally in view of the fact that the cement slurry is pulverized with the thermal contractions, offers a very limited type of protection and consequently is not very appropriate for the purpose of effectively repairing or reinforcing concrete girders for bridges.
On the other hand, in order to reinforce prestressed concrete girders for bridges, the systems utilized up to the present date are even more costly and practically imply the building of an additional super-structure which will support the existent structure, inasmuch as they require a considerable increase in the concrete sections and enormous anchoring diaphragms, together with a relatively large amount of over-reinforcement elements in order to absorb the shearing stresses, and new tendons with sufficient capacity to replace the prior tendons, whereby the cost of repairing is significantly increased and the weight of the structure is increased to a very substantial extent, which may even very frequently necessitate additional reinforcement of the sub-structures.
Consequently, it may be concluded that none of the prior art methods described above has fulfilled the purpose of providing an efficient system to reinforce or repair pre-stressed or reinforced concrete girders for bridges at a reasonable cost and without the need of stopping the traffic which normally runs over the bridges.